Cancer of the colon and rectum continues to kill within 5 years more than 50 per cent of the people in whom it occurs even though we now have all the techniques necessary to provide an early diagnosis. We have embarked on a mass screening program for the early detection of colorectal cancer and for the identification of patients with premalignant polyps, screening with 6 Hemoccult slides on a 3-day, high-bulk, meat-free diet in asymptomatic patients. To date 6135 patients have been screened. Compliance has been 85 per cent. The rate of positive slides has been one per cent. This represents an 0.9% positivity in patients who had previous examinations at the Center, and 1.9% in patients who came for the first time. In most patients only 1 or 2 slides have been positive. Of the 69 patients with positive slides, to date 54 have agreed to further GI studies. Of the 54 who have completed their studies, which included sigmoidoscopy, air-constrast barium enema, and colonoscopy, findings to date are: 26 with adenomatous polyps or villous adenomas; 5 with cancer; 8 with hyperplastic polyps; 10 with diverticulosis; 1 with regional ileitis. Approximately one-third of the polyps were detected by colonoscopy and not visualized by barium enema. Directed endoscopic biopsy and cytology has been accurate in the diagnosis of cancers that were observed, but lavage cytology through the sigmoidoscope or colonoscope has not been especially useful. Cytology, isotope incorporation, and enzyme assays are being studied on endoscopically obtained material. In 200 patients screened with previous colon cancer or polyps we have detected 9 polyps and 2 cancers in 9 patients.